This set of models illustrates variations of planet-sun gear mechanisms. These kinematic chains consist of an inner or 'sun' gear, an outer or 'ring' gear and an intermediate gear called the 'planet.' The models G2, G3, G4, G5, G6 all have elements of such planetary gear trains. Reuleaux calls them 'StirnrÃ¤derwerk' in German. Planetary gears have several natural inversions by grounding or fixing either the sun, ring, or planet gears. There is also a link between the axel of the sun and planet and this can also be grounded. Reuleaux designed several of these models so that they could be demounted and different elements fixed such that one model could demonstrate several different inversions of the same planetary gear kinematic chain.

Planetary gear mechanisms have been used in modern automatic transmission systems of automobiles. They are also used in speed reduction mechanisms in servo motors used in modern robotics and control actuated machines. One Swiss company makes servo motor planetary-gear speed reducers as small as 10 mm in diameter.